Our aim is to determine the number and frequency of alleles segregating at loci coding for proteins and enzymes of different physiological significance. We will specifically examine isogenic lines of second chromosome of Drosophila pseudoobscura for variations at the xanthine dehydrogenase and octanol dehydrogenase loci. Kinetics of xanthine dehydrogenase will be studied in different isogenic strains and we will examine these strains for correlations between kinetics of xanthine dehydrogenase activity and the phenotypes - eye pigment and viability on purine media which are controlled by this enzyme. We will also study variations of loci Pt-10, Pt-12 and amylase which are located on the third chromosome of D. pseudoobscura and D. persimilis. Allelic identifications in all of these cases will be made by electrophoresis in gels of varying concentrations, different pH's and molarities, and by studies of relative activity, thermolability and reactions with thiol reagents. BIBLIOGRAPHIC REFERENCES: McDowell, R.E. and S. Prakash. 1976. Allelic heterogeneity within allozymes separated by electrophoresis in Drosophila pseudoobscura. Proc. Natl. Acad. Sci, U.S.A. 73: 4150-4153. Prakash, S. 1976. Gene differences between third chromosome inversions of Drosophila pseudoobscura. Genetics 84: 787-790.